1. Field of the Invention
The present invention relates generally to measuring magnesium concentrations in molten metal alloys, and, more particularly, to measuring the concentration of magnesium in molten aluminum alloys.
2. Description of Related Art
Great interest exists within the aluminum industry in the possibility of real-time monitors for molten metal processing. Specifically, work is in progress to develop an electrolytic "demag" process for removing magnesium impurities from scrap aluminum. The demag process is designed to reduce the magnesium content in the scrap aluminum to less than 0.1%. A real-time magnesium sensor is needed to determine when this process is complete, since continuing electrolysis to a magnesium concentration much below 0.1% would waste both time and electric power.
A magnesium sensor has been described by B. L. Tiwari, "Electrochemical Probe for Measuring Magnesium Concentration in Molten Aluminum", U.S. Pat. No. 4,601,810, issued Jul. 22, 1986. The sensor of this reference, is based on the electrochemical concentration cell: EQU [Mg(l)].vertline..vertline.[MgCl.sub.2 CaCl.sub.2 (l)].vertline..vertline.[Mg in Al(l)],
where "l" denotes liquid (molten material).
The electrode gives a reliable indication of the magnesium content of molten aluminum and is not appreciably affected by the presence of other alloying elements. The measured electrode potential E varies with magnesium content and temperature exactly as predicted by the Nernst Equation: ##EQU1## where R is the gas constant, T is the absolute temperature, F is the Faraday constant, and [Mg] is the thermodynamic activity of magnesium in the aluminum. However, the sensor was optimized for single use laboratory measurements and is unsuitable for multiple-use, industrial applications. Among the drawbacks are high materials and construction costs, a short operating life, and the loss of contained liquids (electrolyte and magnesium) when the probe is withdrawn from the melt.
A Canadian group developed lithium and sodium ion sensors using (solid) .beta." alumina as the electrolyte; see, A. A. Dubreuil et al, "Solid Electrolyte Probes to Monitor the Alkali and Alkaline Earth Content of Molten Aluminum" in Light Metals, L. G. Boxall, Ed., The Metallurgical Society, Inc., pp. 495-499 (1988). However, the investigators have not been able to extend the approach to magnesium.
Alcoa developed a successful lithium electrode using (solid) fused silica as the electrolyte; see, D. H. DeYoung, "Lithium Sensor for Molten Aluminum-Lithium Alloys", Abstract #763, Fall Meeting of the Electrochemical Society (1990). However, no results for other metals have been published.
Despite extensive research, no practical sensor has yet been commercialized for monitoring magnesium in molten aluminum. Thus, there remains a need for a practical sensor for monitoring the presence of magnesium in molten aluminum.